Communications in a medical device system

ABSTRACT

Systems and methods for communicating between medical devices. In on example, a medical device comprises a communication module for communicating with an implantable leadless cardiac pacemaker through body tissue and a controller operatively coupled to the communications module. The controller may be configured to: identify intrinsic heartbeats; provide a blanking period after each occurrence of an intrinsic heartbeat; and communicate with the implantable leadless cardiac pacemaker via the communication module only during times between the blanking periods.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.62/033,998, filed Aug. 6, 2014, U.S. Provisional Application No.62/033,932, filed Aug. 6, 2014, U.S. Provisional Application No.62/033,978, filed Aug. 6, 2014, and U.S. Provisional Application No.62/034,017, filed Aug. 6, 2014, the complete disclosures of which areherein incorporated by reference.

TECHNICAL FIELD

The present disclosure generally relates to medical devices, and moreparticularly to communications between medical devices in a multi-devicesystem.

BACKGROUND

Pacing instruments can be used to treat patients suffering from variousheart conditions that may result in a reduced ability of the heart todeliver sufficient amounts of blood to a patient's body. These heartconditions may lead to rapid, irregular, and/or inefficient heartcontractions. To help alleviate some of these conditions, variousdevices (e.g., pacemakers, defibrillators, etc.) can be implanted in apatient's body. Such devices may monitor and provide electricalstimulation to the heart to help the heart operate in a more normal,efficient and/or safe manner. In some cases, a patient may have multipleimplanted devices, including devices that are intended to treat otherparts of the body.

SUMMARY

The present disclosure relates generally to systems and methods forcoordinating treatment of abnormal heart activity using multipleimplanted devices within a patient. It is contemplated that the multipleimplanted devices may include, for example, pacemakers, defibrillators,diagnostic devices, and/or any other suitable implantable devices, asdesired. The multiple implanted devices may communicate with each other,for example by sending communication pulses between the devices. In someexamples, a first device may use communication pulses to form messagesfor causing a second device to, as a few non-limiting examples, writedata to one or more data storage modules of the second device, read datafrom one or more data storage modules of the second device, send aresponse message back to the first device, set an address for the seconddevice, or reset the second device. Other messages and message functionsare contemplated by this disclosure.

In a first example, a medical device comprises a communication modulefor communicating with an implantable leadless cardiac pacemaker throughbody tissue; a controller operatively coupled to the communicationsmodule, wherein the controller is configured to: identify intrinsicheartbeats and/or pacing pulses; provide a blanking period after eachoccurrence of an intrinsic heartbeat and each occurrence of a pacingpulse; and communicate with the implantable leadless cardiac pacemakervia the communication module only during times between the blankingperiods.

Alternatively, or additionally, in any of the above examples, themedical device may communicate with the implantable leadless cardiacpacemaker via conducted communication.

Alternatively, or additionally, in any of the above examples, tocommunicate with the implantable leadless cardiac pacemaker viaconducted communication, the controller may be configured to provide aplurality of communication pulses through body tissue via thecommunication module, wherein each of the communication pulses is belowa capture threshold of a heart.

Alternatively, or additionally, in any of the above examples, thecommunication pulses are biphasic communication pulses.

Alternatively, or additionally, in any of the above examples, thecontroller may be further configured to: determine at least onecombination of pulse amplitude and pulse width of communication pulseswhich, when delivered through the body tissue, do not capture the heart;and provide the plurality of communication pulses through body tissuevia the communication module wherein the communication pulses have thedetermined at least one combination of pulse amplitude and pulse width.

Alternatively, or additionally, in any of the above examples, tocommunicate with the implantable leadless cardiac pacemaker viaconducted communication, the controller may be configured to communicateone or more messages of predefined lengths to the implantable leadlesscardiac pacemaker via the communication module by conductedcommunication.

Alternatively, or additionally, in any of the above examples, tocommunicate with the implantable leadless cardiac pacemaker viaconducted communication, the controller may be configured to communicatea first message to the implantable leadless cardiac pacemaker via thecommunication module.

Alternatively, or additionally, in any of the above examples, if anintrinsic heartbeat occurs while the controller is communicating a firstmessage to the implantable leadless cardiac pacemaker via conductedcommunication, the medical device may be configured to provide a firstblanking period and resending the first message to the implantableleadless cardiac pacemaker via the communication module after expirationof the first blanking period.

Alternatively, or additionally, in any of the above examples, if apacing pulse occurs while the controller is communicating the firstmessage to the implantable leadless cardiac pacemaker via conductedcommunication, the medical device may be configured to provide a secondblanking period and resending the first message to the implantableleadless cardiac pacemaker via the communication module after expirationof the second blanking period, wherein the second blanking period islonger than the first blanking period.

Alternatively, or additionally, in any of the above examples, tocommunicate to the implantable leadless cardiac pacemaker, thecontroller may be further configured to receive a second message via thecommunication module by conducted communication in response to the firstmessage.

Alternatively, or additionally, in any of the above examples, if thecontroller fails to receive the second message via the communicationmodule within a predetermined period of time after sending the firstmessage, the controller may be further configured to resend the firstmessage via the communication module by conducted communication.

Alternatively, or additionally, in any of the above examples, thecontroller may be further configured to: provide a first blanking periodafter each intrinsic heartbeat; and provide a second blanking periodafter each pacing pulse, wherein the second blanking period is longerthan the first blanking period.

Alternatively, or additionally, in any of the above examples, themedical device is an implantable medical device.

Alternatively, or additionally, in any of the above examples, themedical device is an implantable subcutaneous cardioverter-defibrillator(S-ICD).

Alternatively, or additionally, in any of the above examples, themedical device is an external medical device.

In another example, a method for communicating between a plurality ofmedical devices, wherein at least one of the plurality of medicaldevices is an implantable medical device, comprises sensing cardiacelectrical signals; determining occurrences of intrinsic heartbeats;providing a blanking period after each occurrence of an intrinsicheartbeat; and allowing communication between a first medical device anda second medical device except during the blanking periods.

Alternatively, or additionally, any of the above examples may furthercomprise determining occurrences of pacing pulses, and providing ablanking period after each occurrence of a pacing pulse.

Alternatively, or additionally, in any of the above examples, the firstmedical device and the second medical device are both implantablemedical devices.

Alternatively, or additionally, in any of the above examples,communication between the first medical device and the second medicaldevice comprises conducted communication through body tissue.

Alternatively, or additionally, in any of the above examples, providinga blanking period after each occurrence of an intrinsic heartbeat andeach occurrence of a pacing pulse comprises providing a first blankingperiod after each occurrence of an intrinsic heartbeat and providing asecond blanking period after each occurrence of a pacing pulse, whereinthe second blanking period is longer than the first blanking period.

Alternatively, or additionally, in any of the above examples,communication between the first medical device and the second medicaldevice comprises communicating one or more messages of predefinedlengths.

Alternatively, or additionally, in any of the above examples,communicating between the first medical device and the second medicaldevice comprises communicating a first message from the first medicaldevice to the second medical device via conducted communication.

Alternatively, or additionally, in any of the above examples, if anintrinsic heartbeat occurs while the first medical device is sending thefirst message from the first medical device to the second medical devicevia conducted communication, providing a first blanking period andresending the first message after expiration of the first blankingperiod.

Alternatively, or additionally, in any of the above examples, if apacing pulse occurs while the first medical device is sending the firstmessage from the first medical device to the second medical device viaconducted communication, providing a second blanking period andresending the first message after expiration of the corresponding secondblanking period, wherein the second blanking period is longer than thefirst blanking period.

Alternatively, or additionally, in any of the above examples,communicating between the first medical device and the second medicaldevice comprises sending a second message from the second medical deviceto the first medical device via conducted communication in response tothe first message.

Alternatively, or additionally, in any of the above examples, if thefirst medical device fails to receive the second message within apredetermined period of time after sending the first message, resendingthe first message.

In yet another example, a medical device comprises a communicationmodule for communicating with an implantable leadless cardiac pacemakerthrough body tissue; and a controller operatively coupled to thecommunication module, wherein the controller is configured to: identifyintrinsic heartbeats; provide a blanking period after each occurrence ofan intrinsic heartbeat; and communicate with the implantable leadlesscardiac pacemaker via the communication module only during times betweenthe blanking periods.

Alternatively, or additionally, in any of the above examples, thecontroller may further be configured to: identify pacing pulses; andprovide a blanking period after each occurrence of a pacing pulse.

Alternatively, or additionally, in any of the above examples, themedical device is an implantable subcutaneous cardioverter.

Alternatively, or additionally, in any of the above examples, thecommunication module communicates with the implantable leadless cardiacpacemaker via conducted communication.

Alternatively, or additionally, in any of the above examples, thecommunication module communicates with the implantable leadless cardiacpacemaker by providing a plurality of communication pulses through bodytissue, wherein each of the communication pulses is below a capturethreshold of a heart.

Alternatively, or additionally, in any of the above examples, thecontroller may be further configured to: provide a first blanking periodafter each intrinsic heartbeat; and provide a second blanking periodafter each pacing pulse, wherein the second blanking period is longerthan the first blanking period.

In still another example, a medical device system configured to deliverelectrical stimulation therapy to a heart of a patient, the systemcomprising: an implantable leadless cardiac pacemaker configured todeliver electrical stimulation therapy to the heart of the patient; amedical device communicatively coupled to the implantable leadlesscardiac pacemaker and configured to: identify intrinsic heartbeats; andinitiate communication messages between the medical device and theimplantable leadless cardiac pacemaker only at times other than duringthe identified intrinsic heartbeats.

Alternatively, or additionally, in any of the above examples, themedical device may be further configured to: identify pacing pulses; andinitiate communication messages between the medical device and theimplantable leadless cardiac pacemaker only at times other than duringthe identified intrinsic heartbeats and the identified pacing pulses.

Alternatively, or additionally, in any of the above examples, themedical device may be further configured to: provide a blanking periodafter each identified intrinsic heartbeat and each identified pacingpulse; and allowing communication messages to be initiated between themedical device and the implantable leadless cardiac pacemaker exceptduring the blanking periods.

Alternatively, or additionally, in any of the above examples, themedical device may be further configured to: provide a first blankingperiod after each identified intrinsic heartbeat; and provide a secondblanking period after each identified pacing pulse, wherein the secondblanking period is longer than the first blanking period.

Alternatively, or additionally, in any of the above examples, if themedical device identifies an intrinsic heartbeat while communicating amessage between the medical device and the implantable leadless cardiacpacemaker, the medical device is further configured to provide a firstblanking period and resending the message after expiration of the firstblanking period.

Alternatively, or additionally, in any of the above examples, if themedical device identifies a pacing pulse while communicating a messagebetween the medical device and the implantable leadless cardiacpacemaker, the medical device is further configured to provide a secondblanking period and resending the message after expiration of the secondblanking period, wherein the second blanking period is longer than thefirst blanking period.

Additionally, it should be understood that any of the above describedmethods may be performed by any of the above described devices and/orsystems. Of course, the methods may also be performed by devices and/orsystems not explicitly described above, but which have the ability toperform the methods as described.

The above summary is not intended to describe each embodiment or everyimplementation of the present disclosure. Advantages and attainments,together with a more complete understanding of the disclosure, willbecome apparent and appreciated by referring to the followingdescription and claims taken in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure may be more completely understood in consideration of thefollowing description of various illustrative embodiments in connectionwith the accompanying drawings, in which:

FIG. 1 illustrates a block diagram of an exemplary leadless cardiacpacemaker (LCP) having electrodes, according to one example of thepresent disclosure;

FIG. 2 illustrates a block diagram of an exemplary medical sensingdevice, according to one example of the present disclosure;

FIG. 3 illustrates a block diagram of an exemplary lead-based medicaldevice, according to one example of the present disclosure;

FIG. 4 illustrates a block diagram of an exemplary external medicaldevice, according to one example of the present disclosure;

FIG. 5 is a schematic diagram of a system including multiple medicaldevices, in accordance with examples of the present disclosure;

FIGS. 6A-D are schematic diagrams illustrating communication pulses, inaccordance with an example of the present disclosure;

FIG. 7 shows an illustrative graph of pulse amplitude vs. pulse width,in accordance with aspects of the present disclosure;

FIG. 8 is a schematic diagram of an example circuit for generatingcommunication pulses, in accordance with one example of the presentdisclosure;

FIG. 9 is a schematic diagram of an example circuit for receivingcommunication pulses, in accordance with one example of the presentdisclosure;

FIG. 10 shows an illustrative timing diagram showing examplecommunication pulses delivered by a medical device in relation to eachother, in accordance with an example of the present disclosure;

FIG. 11 shows an illustrative command message structure, in accordancewith an example of the present disclosure;

FIG. 12 shows an illustrative response message structure, in accordancewith an example of the present disclosure;

FIG. 13 is a flow diagram of an illustrative method that may beimplemented by a medical device or medical device system, such as theillustrative medical devices and medical device systems described withrespect to FIGS. 1-4;

FIG. 14 shows an illustrative timing diagram showing communication of anexample command message in relation to communication of an exampleresponse message, in accordance with an example of the presentdisclosure;

FIG. 15 shows an illustrative timing diagram showing communication ofexample command message and response message pairs in relation a cardiaccycle, in accordance with an example of the present disclosure; and

FIG. 16 is a flow diagram of an illustrative method that may beimplemented by a medical device or medical device system, such as theillustrative medical devices and medical device systems described withrespect to FIGS. 1-5.

While the disclosure is amenable to various modifications andalternative forms, specifics thereof have been shown by way of examplein the drawings and will be described in detail. It should beunderstood, however, that the intention is not to limit aspects of thedisclosure to the particular illustrative embodiments described. On thecontrary, the intention is to cover all modifications, equivalents, andalternatives falling within the spirit and scope of the disclosure.

DESCRIPTION

The following description should be read with reference to the drawingsin which similar elements in different drawings are numbered the same.The description and the drawings, which are not necessarily to scale,depict illustrative embodiments and are not intended to limit the scopeof the disclosure.

A normal, healthy heart induces contraction by conducting intrinsicallygenerated electrical signals throughout the heart. These intrinsicsignals cause the muscle cells or tissue of the heart to contract. Thiscontraction forces blood out of and into the heart, providingcirculation of the blood throughout the rest of the body. However, manypatients suffer from cardiac conditions that affect this contractilityof their hearts. For example, some hearts may develop diseased tissuesthat no longer generate or conduct intrinsic electrical signals. Suchpatients may need a medical device to provide pacing therapy to theirheart in order to cause their heart to contract and pump blood.

FIGS. 1-4 generally depict implantable medical devices that may be usedin systems for delivering pacing therapy, for example including pacingpulses, to a heart of a patient. Some systems may include a plurality ofmedical devices, such as those described with respect to FIGS. 1-4,which may coordinate to deliver pacing therapy to a heart. While medicaldevices configured to deliver therapy to the heart of a patient are usedas an example multi-device system, the present disclosure should not beso limited. Other multi-device systems are contemplated includingsystems that have an implantable neuro-stimulator, an implantablesense-only device, and/or any other suitable medical device as desired.This disclosure describes techniques for communicating between devicesof such multi-device systems.

FIG. 1 depicts an exemplary leadless cardiac pacemaker (LCP) that may beimplanted into a patient and may operate to deliver one or more types ofpacing therapy to the heart of the patient, for example by appropriatelydelivering pacing pulses. In some examples, the LCP may deliver pacingpulses in accordance with one or more therapy techniques, such asbradycardia therapy, rate responsive pacing therapy, anti-tachycardiapacing (ATP) therapy, cardiac resynchronization therapy (CRT),defibrillation therapy, and/or the like. As can be seen in FIG. 1, LCP100 may be a compact device with all components housed within LCP 100 ordirectly on housing 120. As shown in the example of FIG. 1, LCP 100 mayinclude communication module 102, pulse generator module 104, electricalsensing module 106, mechanical sensing module 108, processing module110, battery 112, and electrodes 114.

Communication module 102 may be configured to communicate with devicessuch as sensors, other medical devices, or the like, that are locatedexternally to LCP 100. Such devices may be located either external orinternal to the patient's body. Irrespective of the location, externaldevices (i.e. external to the LCP 100 but not necessarily external tothe patient's body) can communicate with LCP 100 via communicationmodule 102 to accomplish one or more desired functions. For example, LCP100 may communicate information, such as sensed electrical signals,instructions, other messages, and/or data to an external medical devicethrough communication module 102. The external medical device may usethe communicated data and/or messages to perform various functions, suchas determining occurrences of arrhythmias, delivering electricalstimulation therapy, storing received data, and/or other functions. LCP100 may additionally receive instructions, data, and/or other messagesfrom the external medical device through communication module 102, andLCP 100 may use the received instructions, data, and/or other messagesto perform various functions, such as determining occurrences ofarrhythmias, delivering electrical stimulation therapy, storing receiveddata, and/or other functions. Communication module 102 may be configuredto use one or more methods for communicating with external devices. Forexample, communication module 102 may communicate via conductedcommunication signals, radiofrequency (RF) signals, inductive coupling,optical signals, acoustic signals and/or any other signals suitable forcommunication. Illustrative communication techniques between LCP 100 andother devices will be discussed in further detail with reference toother Figures.

In the example shown, pulse generator module 104 may be electricallyconnected to one or more electrodes 114. In some examples, LCP 100 mayadditionally include electrodes 114′. In such examples, pulse generatormodule 104 may additionally be electrically connected to one or moreelectrodes 114′. Pulse generator module 104 may be configured togenerate electrical stimulation signals, such as pacing pulses. Forexample, pulse generator module 104 may generate electrical stimulationsignals by using energy stored in battery 112 within LCP 100 and deliverthe generated electrical stimulation signals to tissues of a patient viaelectrodes 114 and/or 114′. In at least some examples, pulse generatormodule 104 or LCP 100 may further include switching circuitry toselectively connect one or more of electrodes 114 and/or 114′ to pulsegenerator module 104 in order to select via which electrodes 114/114′pulse generator 104 delivers the electrical stimulation signals. Pulsegenerator module 104 may generate electrical stimulation signals withparticular features or in particular sequences in order to provide oneor multiple of a number of different electrical stimulation therapies.For example, pulse generator module 104 may be configured to generateelectrical stimulation signals to provide electrical stimulation therapyto combat bradycardia arrhythmias, tachyarrhythmia arrhythmias,fibrillation arrhythmias, and/or cardiac synchronization arrhythmias. Inother examples, pulse generator module 104 may be configured to generateelectrical stimulation signals to provide electrical stimulationtherapies different than those described herein to treat one or moredetected cardiac arrhythmias.

In some examples, LCP 100 may include electrical sensing module 106 andmechanical sensing module 108. Electrical sensing module 106 may beconfigured to sense the electrical cardiac activity of the heart. Forexample, electrical sensing module 106 may be connected to one or moreelectrodes 114/114′ and electrical sensing module 106 may be configuredto receive electrical cardiac signals conducted through electrodes114/114′. In some examples, the electrical cardiac signals may representlocal information from the chamber in which LCP 100 is implanted. Forinstance, if LCP 100 is implanted within a ventricle of the heart,electrical cardiac signals sensed by LCP 100 through electrodes 114/114′may represent ventricular electrical cardiac signals. Mechanical sensingmodule 108 may include, or be electrically connected to, varioussensors, such as accelerometers, blood pressure sensors, heart soundsensors, blood-oxygen sensors, and/or other sensors which measure one ormore physiological parameters of the heart and/or patient. Bothelectrical sensing module 106 and mechanical sensing module 108 may befurther connected to processing module 110, and may provide signalsrepresentative of the sensed electrical cardiac activity and/orphysiological parameters to processing module 110. Although describedwith respect to FIG. 1 as separate sensing modules, in some examples,electrical sensing module 106 and mechanical sensing module 108 may becombined into a single module.

In some instances, processing module 110 may be configured to controlthe operation of LCP 100. For example, processing module 110 may beconfigured to receive electrical cardiac signals from electrical sensingmodule 106 and/or physiological parameters from mechanical sensingmodule 108. Based on the received signals, processing module 110 maydetermine occurrences and types of arrhythmias. Based on any determinedarrhythmias, processing module 110 may control pulse generator module104 to generate electrical stimulation in accordance with one or moreelectrical stimulation therapies to treat the determined arrhythmias.Processing module 110 may further receive information from communicationmodule 102. In some examples, processing module 110 may use suchreceived information, either instead of or in addition to informationreceived from electrical sensing module 106 and/or mechanical sensingmodule 108, in determining whether an arrhythmia is occurring, indetermining a type of arrhythmia, and/or in determining to takeparticular action in response to the information. Processing module 110may additionally control communication module 102 to send information toother devices.

In some examples, processing module 110 may include a pre-programmedchip, such as a very-large-scale integration (VLSI) chip or anapplication specific integrated circuit (ASIC). In such embodiments, thechip may be pre-programmed with control logic in order to control theoperation of LCP 100. By using a pre-programmed chip, processing module110 may use less power than other programmable circuits while able tomaintain basic functionality, thereby increasing the battery life of LCP100. In other examples, processing module 110 may include a programmablemicroprocessor or the like. Such a programmable microprocessor may allowa user to adjust the control logic of LCP 100 after manufacture, therebyallowing for greater flexibility of LCP 100 than when using apre-programmed chip. In some examples, processing module 110 may furtherinclude a memory circuit and processing module 110 may store informationon and read information from the memory circuit. In other examples, LCP100 may include a separate memory circuit (not shown) that is incommunication with processing module 110, such that processing module110 may read and write information to and from the separate memorycircuit. The memory circuit, whether part of processing module 110 orseparate from processing module 110 may have address lengths of, forexample, eight bits. However, in other examples, the memory circuit mayhave address lengths of sixteen, thirty-two, or sixty-four bits, or anyother bit length that is suitable. Additionally, the memory circuit maybe volatile memory, non-volatile memory, or a combination of bothvolatile memory and non-volatile memory.

Battery 112 may provide a power source to LCP 100 for its operations. Insome examples, battery 112 may be a non-rechargeable lithium-basedbattery. In other examples, the non-rechargeable battery may be madefrom other suitable materials known in the art. Because LCP 100 is animplantable device, access to LCP 100 may be limited. In suchcircumstances, it is necessary to have sufficient battery capacity todeliver therapy over an extended period of treatment such as days,weeks, months, or years. In some examples, battery 110 may arechargeable battery in order to facilitate increasing the useablelifespan of LCP 100.

As depicted in FIG. 1, LCP 100 may include electrodes 114, which can besecured relative to housing 120 but exposed to the tissue and/or bloodsurrounding LCP 100. In some cases, electrodes 114 may be generallydisposed on either end of LCP 100 and may be in electrical communicationwith one or more of modules 102, 104, 106, 108, and 110. In someexamples, LCP 100 may additionally include one or more electrodes 114′.Electrodes 114′ may be positioned on the sides of LCP 100 and increasethe number of electrodes by which LCP 100 may sense electrical cardiacactivity and/or deliver electrical stimulation. Electrodes 114 and/or114′ can be made up of one or more biocompatible conductive materialssuch as various metals or alloys that are known to be safe forimplantation within a human body. In some instances, electrodes 114and/or 114′ connected to LCP 100 may have an insulative portion thatelectrically isolates the electrodes 114 from adjacent electrodes,housing 120, and/or other materials. In some cases, electrodes 114and/or 114′ may be spaced from the housing and connected throughconnecting wires. In such embodiments, the electrodes 114 and/or 114′may be placed on a on a tail that extends from the housing 120.

It is contemplated that electrodes 114 and/or 114′ may have any of avariety of sizes and/or shapes, and may be spaced at any of a variety ofdistances. For example, electrodes 114 may have a diameter of two totwenty millimeters (mm). However, in other examples, electrodes 114and/or 114′ may have a diameter of two, three, five, seven millimeters(mm), or any other suitable diameter, dimension and shape. In somecases, electrodes 114 and/or 114′ may have a length of zero, one, three,five, ten millimeters (mm), or any other suitable length, where thelength is a dimension of electrodes 114 and/or 114′ that extends awayfrom housing 120. Additionally, at least some of electrodes 114 and/or114′ may be spaced from one another by a distance of twenty, thirty,forty, fifty millimeters (mm), or any other suitable distance. Theelectrodes 114 and/or 114′ of a single device may have different sizeswith respect to each other, and the spacing of the electrodes on thedevice may not be uniform.

To implant LCP 100 inside a patient's body, an operator (e.g., aphysician, clinician, etc.), may fix LCP 100 to the cardiac tissue ofthe patient's heart. To facilitate fixation, LCP 100 may include one ormore anchors 116. Anchor 116 may include any number of fixation oranchoring mechanisms. For example, anchor 116 may include one or morepins, staples, threads, screws, helix, tines, and/or the like. In someexamples, although not shown, anchor 116 may include threads on itsexternal surface that may run along at least a partial length of anchor116. The threads may provide friction between the cardiac tissue and theanchor to help fix anchor 116 within the cardiac tissue. In otherexamples, anchor 116 may include other structures such as barbs, spikes,or the like to facilitate engagement with the surrounding cardiactissue.

FIG. 2 depicts an exemplary medical device, MD 200, which may beimplanted into a patient and may operate to sense one or more signalsrepresentative of a physiological condition of the patient. As can beseen in FIG. 2, MD 200 may be a compact device with all componentshoused within MD 200 or directly on housing 220. As illustrated in FIG.2, MD 200 may include communication module 202, electrical sensingmodule 206, mechanical sensing module 208, processing module 210,battery 212, and electrodes 214/214′.

In some examples, MD 200 may be similar to LCP 100 as described withrespect to FIG. 1. For example, communication module 202, electricalsensing module 206, mechanical sensing module 208, processing module210, battery 212, and electrodes 214/214′ may be similar tocommunication module 102, electrical sensing module 106, mechanicalsensing module 108, processing module 110, battery 112, and electrodes114/114′, as described with respect to FIG. 1. However, MD 200 may notinclude a pulse generator module. For instance, MD 200 may be adedicated sensor device. Accordingly, in some examples, MD 200 may bethe same as LCP 100 with a few minor hardware differences.Alternatively, MD 200 may include all of the components of LCP 100,except that one or more of the components may be disabled or not used,such as a pulse generator module.

In other examples, MD 200 may include substantially different hardwarethan LCP 100. For instance, MD 200 may be substantially different insize than LCP 100, as MD 200 may not require as severe of sizeconstraints as LCP 100 due to typical implant locations for MD 200. Insuch examples, MD 200 may include a larger battery and/or more powerfulprocessing unit than LCP 100.

FIG. 3 depicts an example of another device, medical device (MD) 300,which may be used in conjunction with LCP 100 of FIG. 1 in order todetect and treat cardiac arrhythmias and other heart conditions. In theexample shown, MD 300 may include a communication module 302, a pulsegenerator module 304, an electrical sensing module 306, a mechanicalsensing module 308, a processing module 310, and a battery 318. Each ofthese modules may be similar to modules 102, 104, 106, 108, and 110 ofLCP 100. Additionally, battery 318 may be similar to battery 112 of LCP100. However, in some examples, MD 300 may have a larger volume withinhousing 320. In such examples, MD 300 may include a larger batteryand/or a larger processing module 310 capable of handling more complexoperations than processing module 110 of LCP 100.

While MD 300 may be another leadless device such as shown in FIG. 1, insome instances MD 300 may include leads, such as leads 312. Leads 312may include electrical wires that conduct electrical signals betweenelectrodes 314 and one or more modules located within housing 320. Insome cases, leads 312 may be connected to and extend away from housing320 of MD 300. In some examples, leads 312 are implanted on, within, oradjacent to a heart of a patient. Leads 312 may contain one or moreelectrodes 314 positioned at various locations on leads 312 and variousdistances from housing 320. Some leads 312 may only include a singleelectrode 314, while other leads 312 may include multiple electrodes314. Generally, electrodes 314 are positioned on leads 312 such thatwhen leads 312 are implanted within the patient, one or more of theelectrodes 314 are positioned to perform a desired function. In somecases, the one or more of the electrodes 314 may be in contact with thepatient's cardiac tissue. In other cases, one or more of the electrodes314 may be subcutaneously implanted but adjacent to the patient's heart.Electrodes 314 may conduct intrinsically generated electrical cardiacsignals to leads 312. Leads 312 may, in turn, conduct the receivedelectrical cardiac signals to one or more of the modules 302, 304, 306,and 308 of MD 300. In some cases, MD 300 may generate electricalstimulation signals, and leads 312 may conduct the generated electricalstimulation signals to electrodes 314. Electrodes 314 may then conductthe electrical stimulation signals to the cardiac tissue of the patient(either directly or indirectly).

Leads 312 may additionally contain one or more sensors, such asaccelerometers, blood pressure sensors, heart sound sensors,blood-oxygen sensors, and/or other sensors which are configured tomeasure one or more physiological parameters of the heart and/orpatient. In such examples, mechanical sensing module 308 may be inelectrical communication with leads 312 and may receive signalsgenerated from such sensors.

While not required, in some examples MD 300 may be an implantablemedical device. In such examples, housing 320 of MD 300 may be implantedin, for example, a transthoracic region of the patient. Housing 320 maygenerally include any of a number of known materials that are safe forimplantation in a human body and may, when implanted, hermetically sealthe various components of MD 300 from fluids and tissues of thepatient's body.

In some cases, MD 300 may be an implantable cardiac pacemaker (ICP). Inthese examples, MD 300 may have one or more leads, for example leads312, which are implanted on or within the patient's heart. The one ormore leads 312 may include one or more electrodes 314 that are incontact with cardiac tissue and/or blood of the patient's heart. MD 300may be configured to sense intrinsically generated cardiac electricalsignals and determine, for example, one or more cardiac arrhythmiasbased on analysis of the sensed signals. MD 300 may be configured todeliver CRT, ATP therapy, bradycardia therapy, and/or other therapytypes via leads 312 implanted within the heart. In some examples, MD 300may additionally be configured to provide defibrillation therapy.

In some instances, MD 300 may be an implantablecardioverter-defibrillator (ICD). In such examples, MD 300 may includeone or more leads implanted within a patient's heart. MD 300 may also beconfigured to sense electrical cardiac signals, determine occurrences oftachyarrhythmias based on the sensed electrical cardiac signals, anddeliver defibrillation therapy in response to determining an occurrenceof a tachyarrhythmia. In other examples, MD 300 may be a subcutaneousimplantable cardioverter-defibrillator (S-ICD). In examples where MD 300is an S-ICD, one of leads 312 may be a subcutaneously implanted lead. Inat least some examples where MD 300 is an S-ICD, MD 300 may include onlya single lead which is implanted subcutaneously but outside of the chestcavity, however this is not required.

In some examples, MD 300 may not be an implantable medical device.Rather, MD 300 may be a device external to the patient's body, andelectrodes 314 may be skin-electrodes that are placed on a patient'sbody. In such examples, MD 300 may be able to sense surface electricalsignals (e.g. cardiac electrical signals that are generated by the heartor electrical signals generated by a device implanted within a patient'sbody and conducted through the body to the skin). In such examples, MD300 may be configured to deliver various types of electrical stimulationtherapy, including, for example, defibrillation therapy.

FIG. 4 depicts an example of another device, medical device (MD) 400,which may be used in conjunction with LCP 100 of FIG. 1 and/or othermedical devices in order to detect and treat cardiac arrhythmias and/orother heart conditions. In the example shown, MD 400 may include acommunication module 402, a pulse generator module 404, an electricalsensing module 406, a mechanical sensing module 408, a processing module410, and a power source 418. Each of these modules may be similar tomodules 102, 104, 106, 108, and 110 of LCP 100. However, MD 400 may bean external medical device. Accordingly, in some examples, power source418 may be a power converter that converts externally supplied power,for example from a wall outlet, into a form suitable for MD 400.

MD 400 may additionally include display 416 connected to processingmodule 410. Display 416 may be a monitor or other screen which iscapable of displaying letters, numbers, graphics, and other forms ofinformation. In at least some examples, display 416 may be able toreceive user input. For example, display 416 may be a touch sensitivedisplay. In other examples, MD 400 may include one or more peripheralinput devices, such as a mouse and/or keyboard. It is contemplated thatthe display 416 may be incorporated into a common housing with MD 400,or may be in a separate housing.

MD 400 may include electrodes 414. In examples where MD 400 is anexternal medical device, electrodes 414 include skin patch electrodes.When electrodes 414 are connected to the skin of a patient, MD 400 maysense electrical signals generated within the patient. In an examplewhere MD 400 includes a pulse generator module 404, MD 400 mayadditionally be able to deliver electrical pulses to the patient throughelectrodes 414. For example, pulse generator module 404 of MD 400 may beconfigured to generate electrical stimulation pulses in accordance withone or more electrical stimulation therapies, which are conductedthrough electrodes 414. Additionally, communication module 402 may beconfigured to generate conducted communication signals, which areconducted through electrodes 414 and into the body. Mechanical sensingmodule 408 may include or be connected either directly orcommunicatively to one or more sensing devices, such as accelerometers,blood pressure sensors, heart sound sensors, blood-oxygen sensors, andother sensors which measure physiological parameters of the heart and/orpatient.

In some examples, MD 400 may be a programming device for programming oneor more other medical devices, such as those depicted in FIGS. 1-3. Insome of these examples, MD 400 may not be configured to deliverelectrical stimulation therapies. A user may enter one or moreparameters into external display 416 and/or another peripheral device,which sends the entered parameters to processing module 410. In at leastsome examples, MD 400 may be used to issue an ID (pairing) command to animplantable medical device, as described subsequently with respect toTable 1. Processing module 410 may instruct communication module 402 tocommunicate the received parameters, or other parameters, to othermedical devices using one or more forms of communication, such asconducted communication signals, radiofrequency (RF) signals, inductivecoupling, optical signals, acoustic signals, and/or any other suitablesignals. Various conducted communication techniques are described hereinwhich communication modules 402 may employ in communicating suchparameters and/or other information.

FIG. 5 illustrates a patient 540 connected to medical device system 500including devices such as those described with respect to FIGS. 1-4.FIG. 5 illustrates the devices of system 500 implanted or positioned atvarious example locations. For instance, LCPs 502, 504, 506 are alldepicted implanted within a different chamber of heart 550. In someexamples, however, the heart may include multiple LCPs implanted withina single chamber, or other LCPs implanted on an external part of heart550. In still other examples, LCPs may be implanted in other chambers ofheart 550 or in different combinations of chambers of heart 550. FIG. 5also depicts LCP 518 implanted at a location remote from heart 550. AnIMD 508 may be a device similar to those described with respect to FIG.3, such as an ICD, or an S-ICD, with lead 509 connected to electrodes510 and implanted subcutaneously. A sensor 516 is depicted implantednear the chest of patient 540, and in some cases, may be similar to MD200 as described with respect to FIG. 2. Sensor 516 may also beimplanted at a location remote from the heart. External medical device512 may not be an implanted medical device. Rather, external medicaldevice 512 may be connected to patient 540 through skin-patch electrodes514 or the like, and may be similar to MD 400 as described with respectto FIG. 4.

Examples of remote locations for LCP 518 and sensor 516 include devicesimplanted in the cephalic, cervical, pectoral, thoracic, abdominal,upper limb and lower limb regions of the patient 500. Additionally,remote locations include implant sites within or on an organ or bodystructure as such locations within or on organs such as the brain, lung,mouth, esophagus, stomach, liver, gallbladder, kidney, pancreas, spleen,intestine, colon, adrenal gland, bladder, uterus, diaphragm, bone.Remote locations also include implant sites in vessels such as bloodvessels (e.g. veins, arteries), lymphatic vessels (e.g. jugular trunk,intestinal trunk) and airway vessels (e.g. trachea, bronchi).

The devices of system 500 may communicate via a communication pathway,for example sending and receiving data, instructions, messages and/orother information. Although it is contemplated that the devices maycommunicate using a variety of modalities, such as with RF signals,inductive coupling, optical signals, or acoustic signals, in at leastsome examples, the devices of system 500 may communicate using conductedcommunication. Accordingly, the devices of system 500 may havecomponents that allow for such conducted communication. As discussedabove with respect to FIGS. 1-4, the devices of system 500 may each havea communication module. Each communication module may be configured togenerate conducted communication signals and transmit the signals intothe patient's body via one or more coupled electrodes, such aselectrodes 502 a, 502 b, 504 a, 504 b, 506 a, 506 b, 510, 514, 518 a,and 518 b. Although not depicted specifically in FIG. 5, sensor 516 mayalso include one or more electrodes. The communication modules mayadditionally be configured to receive conducted communication signalsvia the one or more electrodes. In some examples, devices may use apulse generator module to generate conducted communication signalsinstead of a communication module.

The patient's body tissue may conduct the conducted communicationsignals from the transmitting device to a receiving device. In somecases, the conducted communication signals may be galvanically conductedcommunication signals. For example, a sending device may differentiallycouple conducted communication signals into the body tissue of patient540, and the body tissue acts as a transmission line. The receivingdevice or devices may pick-up these differential signals. This techniqueis in contrast to capacitive techniques, where transmitted and receivedsignals are referenced to a common ground source.

The conducted communication signals, described in more detail withrespect to FIG. 6, may differ from pacing pulses or other electricalstimulation therapy signals. For example, the devices of system 500 maydeliver conducted communication signals at an amplitude/pulse widthcombination that is sub-threshold to the heart so as to not capture theheart. In some cases, the amplitude/pulse width combination of thedelivered conducted communication signals may be above the capturethreshold of the heart, but may be delivered during a refractory periodof the heart and/or may be incorporated in or modulated onto a pacingpulse, if desired.

The conducted communication signals may be voltage pulses, currentpulses, biphasic voltage pulses, biphasic current pulses, or any othersuitable electrical pulse as desired. In some examples, the conductedcommunication signals may be combinations of voltage pulses and currentpulses. Accordingly, in examples where the conducted communicationsignals include voltage pulses, the devices of system 500 may includeappropriate circuitry, such as in a communication module or a pulsegenerator module, for generating voltage pulses. When generating avoltage pulse, the amplitude of the voltage is controlled, and theamplitude of the current is dependent on the voltage amplitude and theresistance of the transmission medium. In examples where the conductedcommunication signals include current pulses, the devices of system 500may include appropriate circuitry for generating current pulses. Whengenerating a current pulse, the amplitude of the current is controlled,and the amplitude of the voltage is dependent on the current amplitudeand the resistance of the transmission medium. In examples where theconducted communication signals comprise both voltage and currentpulses, the devices of system 500 may include appropriate circuitry forgenerating both voltage pulses and current pulses. Some example featuresof conducted communication signals that the devices of system 500 mayuse are described with respect to FIG. 6.

The conducted communication signals may be modulated in any suitablemanner to encode communicated information. For example, and in somecases, the conducted communication signals may be pulse width modulated.Alternatively, or additionally, the time between successive conductedcommunication signals may be modulated to encode desired information.Illustrative techniques for encoding information with conductedcommunication signals and sending messages between devices are describedwith respect to FIGS. 10-15.

FIGS. 6A-6D illustrate some example features of conducted communicationsignals that devices of system 500 may use when communicating. Althoughthe examples are described with respect to conducted voltage signals, itis contemplated that the devices of system 500 may use conducted currentsignals.

FIG. 6A depicts an example communication voltage pulse that devices ofsystem 500 may use in a conducted communication scheme. Specifically,FIG. 6A depicts communication voltage pulse 602, which has a voltageamplitude 604 and pulse width 606. Communication voltage pulse 602 is amonophasic, positive polarity communication voltage pulse. In suchexamples, amplitude 604 may be three, four, or five volts, or any othersuitable amplitude. In some instances, amplitude 604 may be correlatedto the voltage of the battery of the device that generates the voltagepulse. For example, amplitude 604 may between one and two times thevoltage of the battery of the generating device. If the voltage of thebattery of the generating device is six volts, then amplitude 604 may bebetween six and 12 volts. A voltage multiplier (not shown) may be usedto multiply the voltage of the battery for use in generating thecommunication pulses. Pulse width 606 may be one, five, ten, fifteen,twenty microseconds, or any other suitable length of time.

FIG. 6B depicts another example communication voltage pulse that devicesof system 500 may use in a conducted communication scheme. FIG. 6Bdepicts communication voltage pulse 610, which has a voltage amplitude612 and pulse width 614. In contrast with communication voltage pulse602, communication voltage pulse 610 is a monophasic, negative polaritycommunication voltage pulse. That is, amplitude 612 is negative. Forinstance, amplitude 612 may be negative three, negative four, ornegative five volts, or any other suitable amplitude. Pulse width 614may be one, five, ten, fifteen, twenty microseconds, or any othersuitable length of time. As with amplitude 604, in some examples,amplitude 612 may be correlated to a battery voltage of the device thatgenerates the voltage pulse.

FIGS. 6C and 6D both depict other examples of communication voltagepulses that devices of system 500 may use in a conducted communicationscheme. FIG. 6C depicts communication voltage pulse 620, which is abiphasic communication voltage pulse beginning with positive portion 620a and ending with negative portion 620 b. Each of positive portion 620 aand negative portion 620 b have individual amplitudes and pulse widths.Amplitudes 626 and 628 may have a magnitude of three, four, or fivevolts, or any other suitable amplitude, with amplitude 626 having apositive value and amplitude 628 having a negative value. Additionally,in some examples, amplitudes 626 and 628 may be correlated to a batteryvoltage of the device that generates the voltage pulse. Pulse widths 622and 624 may each be one, five, ten, fifteen, twenty microseconds, or anyother suitable length of time. Accordingly, some example total pulsewidths of communication voltage pulse 620 may be two, ten, twenty,thirty, forty microseconds, or any other suitable length of time. FIG.6D depicts communication pulse 640, including negative portion 640 a,positive portion 640 b, pulse widths 642 and 644, and amplitudes 646 and648. Communication voltage pulse 640 is a biphasic communication voltagepulse similar to communication voltage pulse 620, except that voltagepulse 640 has negative portion 640 a preceding positive portion 640 b.Pulse widths 642 and 644 and amplitudes 646 and 648 may have similarvalues to those described for communication voltage pulse 620, ordifferent values.

In some examples, the communication voltage pulses depicted in FIGS.6A-6D may have amplitudes and pulse widths that vary, either betweenpulses or between a positive and negative portion of a pulse. Forinstance, where a device generates multiple monophasic communicationvoltage pulses, the first communication voltage pulse may have a firstset of characteristics, in terms of amplitude and pulse width, and asecond communication voltage pulse may have a second set ofcharacteristics, where at least some of the second set ofcharacteristics differs from the first set of characteristics. In someinstances, the polarity between successive monophasic communicationvoltage pulses may also vary. Although not shown, in some examples theremay be a delay between biphasic pulses. For example, in FIGS. 6C and 6Dthere may be a delay between pulses 620 a and 620 b or 640 a and 640 brespectively. The delay may be one, two, five, ten microseconds, or anyother suitable length of time.

In examples where a medical device generates biphasic communicationvoltage pulses, the amplitude of the first portion of the communicationvoltage pulse may differ from the second portion of the communicationvoltage pulse. Additionally, the pulse width of the first portion of thecommunication voltage pulse may be different than the second portion ofthe communication voltage pulse. Though, the characteristics, in termsof amplitude and pulse width, and even polarity, may differ betweensuccessive biphasic communication voltage pulses instead of or inaddition to differing between different portions of the same biphasiccommunication voltage pulse.

As discussed above, devices of system 500 may generate communicationvoltage pulses that are sub-threshold voltage pulses—voltage pulseswhich do not capture the heart. This may allow the devices of system 500to communicate over a broad range of the cardiac cycle withoutinterfering with any delivery of electrical stimulation therapy, forexample by causing undesirable capture of the heart. Accordingly, theconducted communication voltage pulses used by the devices of system 500may generally have characteristics that fall within a safe region 710,as depicted in FIG. 7.

FIG. 7 shows graph 700, which is a graph of pulse amplitude vs. pulsewidth, in millivolts and milliseconds. Curve 702 represents thecombinations of pulse amplitudes and pulse widths of a voltage pulsethat, when delivered to tissues of a patient, result in capture of thepatient's heart. In this Figure, any combinations of pulse amplitude andpulse width that lie on curve 702, or above and to the right of curve702, have been determined to capture a heart in animal test models. Anycombinations of pulse amplitude and pulse width that lies below and tothe left of curve 702 were determined to not result in capture of theheart. This region is defined as safe zone 710.

In human patients, curve 702 may vary by patient, and is somewhat afunction of time and/or other factors. Accordingly, the exactcombinations or pulse amplitudes and pulse widths that result in captureand do not result in capture may vary, resulting in someunpredictability with respect to whether a given combination of pulseamplitude and pulse width will capture the heart. In some examples,then, safe region 710 may be the combinations of pulse amplitudes andpulse widths that lie below and to the left of a second curve, curve708. Second curve 708 may be of a similar shape as curve 702, onlyshifted down and to the left by a safety margin 706. Safety margin 706may be set such that if curve 702 does change as a function of time orother factors, curve 702 will not, or is statistically unlikely to,drift below and to the left of curve 708. Accordingly, in some examples,safe region 710 may encompass the combinations of pulse amplitudes andpulse widths below and to the left of curve 708, rather than curve 702.

Consequently, the devices of system 500 may be configured to generatecommunication voltage pulses with characteristics within safe region710. In some examples, safe region 710 may be predetermined for aparticular patient, and the devices of system 500 may be configured togenerate communication voltage pulses with a combination of pulseamplitude and pulse width that falls within predetermined safe region710. In some instances, one or more of the devices of system 500 may beconfigured to determine safe region 710 by generating a plurality ofvoltage pulses with different pulse amplitude and pulse widthcharacteristics and determining whether the generated voltage pulsescapture the heart. In these examples, the devices of system 500 may beconfigured to periodically determine one or more combinations of pulseamplitudes and pulse widths which result in capture of the heart. Afterdetermining which combinations of characteristics of voltage pulsesresult in capture, the devices of system 500 may be configured to onlygenerate communication voltage pulses that have lower pulse amplitudesand/or shorter pulse widths than those voltage pulses which resulted incapture. The devices of system 500 may alternatively be configured togenerate communication voltage pulses with characteristics that are apredetermined amount less than and/or shorter than the characteristicsof those voltage pulses that resulted in capture, as a margin of safety.In some instances, the devices of system 500 may be configured togenerate voltage pulses within a composite safe region 710 that ispredetermined based on determined safe regions for a population ofpeople.

FIG. 8 is a diagram of an example circuit 800 that a device of system500 may use to generate communication voltage pulses. In the exampleshown, circuit 800 may be a part of a communication module. Or, inexamples where a pulse generator module generates communication voltagepulses, circuit 800 may be a part of a pulse generator module. Circuit800 may include double switches 802 a and 802 b connected to a firstelectrode and double switches 804 a and 804 b connected to a secondelectrode. The illustrative circuit 800 additionally includes voltagesource 806. The device using circuit 800 may operate switches 802 a, 802b and 804 a, 804 b in a manner that produces one or more of thecommunication voltage pulses described herein. For example, the devicemay close switches 802 a and 804 b, and open switches 802 b and 804 a,to produce a positive amplitude communication pulse between Electrode-Aand Electrode-B. Conversely, the device may close switches 802 b and 804a, and open switches 802 a and 804 b, to produce a negative amplitudecommunication pulse between Electrode-A and Electrode-B. To produce abiphasic communications pulse, the device may close switches 802 a and804 b, and open switches 802 b and 804 a, to produce a positiveamplitude communication pulse between Electrode-A and Electrode-B, andthen immediately, or after a predetermined delay, close switches 802 band 804 a, and open switches 802 a and 804 b, to produce a negativeamplitude communication pulse between Electrode-A and Electrode-B. Thetime that the switches remain in the closed state will determine thecorresponding pulse width. In general, the device may operate theswitches 802 a, 802 b, 804 a, and 804 b in any manner to produce avariety of different communication voltage pulses, such as thosedescribed with respect to FIGS. 6A-6D.

FIG. 9 is a schematic diagram of an illustrative circuit 900 that adevice of system 500 may be used to sense for communication voltagepulses. For instance, circuit 900 may be included in a communicationmodule of a device. Illustrative circuit 900 depicts two inputsconnected to positive and negative terminals of operational amplifier908. The inputs, for example a first and second electrode, are connectedto switches 902 and 904, respectively. The switches 902 and 904 aretypically switched together, and may be used to control when the circuit900 senses for communication pulses. For example, the switches 902 and904 may be opened when a pacing pulse is expected to be delivered, whena shock is expected to be delivered, when an intrinsic heartbeat isexpected to occur, and/or at other times.

The first input of the circuit 900 may be coupled to a positive input ofoperational amplifier 908 through one or more circuit elements. In atleast one example, the circuit elements may include a capacitor 906 aand resistor 906 c. In such examples, capacitor 906 a and resistor 906 cmay operate as a high-pass filter before the signal is fed into thepositive terminal of amplifier 908, thereby attenuating low frequencysignals. In a similar fashion, the second input may be coupled to thenegative input of operational amplifier 908 through one or more circuitelements. In the example of FIG. 9, the second input is coupled to thenegative input of operational amplifier 908 by capacitor 906 b andresistor 906 d. Capacitor 906 b and resistor 906 d may function tooperate as a high-pass filter before the signal is fed into the negativeterminal of operational amplifier 908, thereby attenuating low frequencysignals.

Receiver circuit 950, which includes operational amplifier 908, mayreceive the signals from two electrodes as described above. As thesignal passes through receiver circuit 950, the various elements maycooperate to amplify and/or filter the differential signal to reducenoise and/or enhance features of any communication voltage pulsespresent in the signal. The signal may then exit receiver circuit 950 asan amplified and/or filtered signal at 940. The amplified and/orfiltered signal may then be fed into a processor or other circuit whichmay detect one or more communication voltage pulses.

Receiver circuit 950 may contain one or more amplifiers and/or filteringelements. For example, receiver circuit 950 may contain amplifiers 920and 930. More specifically, the output of amplifier 908 may be fed intothe positive terminal of amplifier 920. The output of amplifier 920 maybe modified by one or more circuit elements 925 before being fed-back tothe negative terminal of amplifier 920. The output of amplifier 920 mayalso be fed into the positive terminal of amplifier 930, and a signalfrom a digital-to-analog converter may be fed into the negative terminalof amplifier 930. The output of amplifier 930, then, may be theamplified and/or filtered signal that is output of receiver circuit 950at 940.

In at least some examples, the devices of system 500 may be constantlyreceiving and processing signals. For instance, switches 902 and 904 maybe constantly closed, conducting sensed signals into circuit 900. Inother examples, the devices of system 500 may be receiving andprocessing signals at least a majority of the time (e.g. for a majorityof each cardiac cycle). Accordingly, circuit 900 may be designed to below power in order improve battery life. In some examples, circuit 900may be designed to have a sensitivity of one millivolt or less with alinear input range of 1-to-100 millivolts, but this is just one example.Circuit 900 may be configured for a source impedance of between 300 and1500 ohms, but again this is just one example.

In some cases, the devices of system 500 may use the elapse time betweencommunication voltage pulses to encode information. FIG. 10 providessome example techniques for encoding information using the elapse timebetween communication voltage pulses. FIG. 10 shows a graph of fourexample communication voltage pulses 1010 a-1010 d. Communicationvoltage pulses 1010 a-1010 d are separated by three distinct timeperiods, 1002, 1004, and 1006, respectively. In the example shown, thelast time period 1008 does not separate one communication voltage pulse1010 d from another communication voltage pulse. Rather, time period1008 is simply a threshold length of time extending from communicationvoltage pulse 1010 d, without a subsequent communication voltage pulse1010 occurring before the end of the threshold length of time. In somecases, the devices of system 500 may identify communication symbolsbased on the length of the time between the communication voltage pulses1010 a-1010 d. For example, if the time between two communicationvoltage pulses falls within a first time range, then a first symbol maybe identified. If the time between two communication voltage pulsesfalls within a second time range, then a second symbol may beidentified. If the time between two communication voltage pulses fallswithin a third time range, then a third symbol may be identified, and soon. In one example, a sync symbol is identified when the time betweentwo communication voltage pulses falls within a range of 800-1100microseconds, a “1” symbol is identified when the time between twocommunication voltage pulses falls within a range of 550-700microseconds, and a “0” symbol is identified when the time between twocommunication voltage pulses falls within a range of 350-450microseconds. In some cases, the “0” and “1” symbols correspond to “0”and “1” bits, respectively, as the devices of system 500 may operate ina base two number system. These are just some examples. It iscontemplated that any number of different symbols may be included in thecommunication protocol, with different symbols assigned to differenttimes or time ranges. In some case, if a communication voltage pulse isnot followed by another communication voltage pulse within a thresholdamount of time (e.g. time period 1008), an end or frame (EOF) symbol maybe identified. The threshold amount of time (e.g. time period 1008) maybe, for example, 1250 microseconds or more.

In some cases, the time between communication voltage pulses may betracked using an internal clock. It is contemplated that the sendingdevice may include an internal clock that oscillates at a clockfrequency. Likewise, the receiving device may include an internal clockthat oscillates at the same (or different) clock frequency. When soprovided, each symbol to be communicated may be assigned a differentnumber of clock cycles between communication voltage pulses. Forexample, a synchronization symbol may be assigned 24 clock cycles, whichfor a clock frequency of 25.6 kHz, would correspond to a delay betweencommunication voltage pulses of about 938 microseconds. A range may beprovided to help compensate for noise, temperature changes, voltagevariances, clock drift, etc. The range may be, for example, +/−10%, orin the example given above, may be from about 844 microseconds to about1032 microseconds. A “1” symbol may be assigned to 16 clock cycles,which for a clock frequency of 25.6 kHz, would corresponds to a delaybetween communication voltage pulses of about 625 microseconds. A rangemay be provided around this figure to help compensate for noise,temperature changes, voltage variances, clock drift, etc. The range maybe, for example, +/−10%, or in the example given above, may be fromabout 563 microseconds to about 688 microseconds. Likewise, a “0” symbolmay be assigned to 10 clock cycles, which for a clock frequency of 25.6kHz, would correspond to a delay between communication voltage pulses ofabout 391 microseconds. A range may be provided around this figure tohelp compensate for noise, temperature changes, voltage variances, clockdrift, etc.

To transmit a desired symbol, the sending device may provide a firstcommunication voltage pulse, then count the number of clock cycles thatcorresponds to the desired symbol (e.g. 16 clock cycles for a “1”symbol), and then provide a second communication pulse. When thereceiving device receives the first communication voltage pulse, thereceiving device may start counting internal clock cycles. When thesecond communication pulse is received, the receiving device may stopcounting clock cycles. The receiving device may then compare the numberof counted internal clock cycles to the number of clock cycles assignedto each symbol. When a match is found, the desired symbol is identifiedby the receiving device.

In some cases, the accuracy of the internal clocks in the sending deviceand/or receiving device may degrade over time. Due to this degradation,the devices of system 500 may begin to determine lengths of timedifferently with respect to absolute lengths of time, and possibly withrespect to each other if the clocks of the devices degrade differentlywith respect to each other. Accordingly, in examples where time periods1002, 1004, 1006, and 1008 are ranges of times, the devices of system500 may still correctly interpret symbols even after some level of clockdegradation.

In some cases, the devices of system 500 may be configured toreconfigure their internal clocks on a periodic or other basis. Forexample, a first device may broadcast a beginning calibration signal, anending calibration signal, and the length of time between the twosignals as determined by the broadcasting device. Each other device maythen calibrate their internal clocks so that the time period between thetwo calibration signals is equal to the length of time sent by thebroadcasting device. Such a reconfiguration may help ensure that a clockof a device does not drift too far relative to that of other devices ofthe system such that the device becomes functionally inoperative.

Whatever the exact lengths of time periods 1002, 1004, 1006, and 1008,in some examples, time period 1002 may be longer than either of timeperiods 1004 and 1006. In such examples, this arrangement may preventaccidental transmission of one or more symbols from one device toanother device, or a device interpreting noise as communication of oneor more symbols. For instance, the devices of system 500 may transmitcommunication voltage pulses to communicate a synchronization symbolbefore transmitting one or more other symbols, and receiving devices mayignore any other symbols received before receiving a synchronizationsymbol. In some situations, a receiving device may receive a first, truecommunication voltage pulse but then receive noise after a length oftime shorter than time period 1002. If the noise is similar inmorphology to a communication voltage pulse, the receiving device mayinterpret the noise as a communication voltage pulse. However, since thenoise occurred after a shorter length of time than time period 1002,even if the noise occurred at a length of time indicating a “0” symbolor a “1” symbol, the receiving device would ignore those symbols as thereceiving device had not yet received a synchronization symbol. In thismanner, the devices of system 500 may suppress erroneously transmittedor falsely interpreted symbols.

In some instances, a blanking period may be applied by the receivingdevice immediately following receiving each communication voltagepulses. During the blanking period, the receiving device may ignore anyreceived communication signals. This may help further reduce noise thatmight arise immediately after a communication voltage pulse from beinginterpreted as a valid communication voltage pulse. The blanking periodmay be anywhere between one-quarter to three-quarters the length of timeperiod 1002, or any other suitable length of time. In one example, theblanking period may be, for example about 250 microseconds. In somecases, the sending device may apply a similar blanking period, duringand/or following the transmission of a communication voltage pulse. Sucha blanking period may help prevent the circuitry of the sending device,which senses for conducted communication signals from other devices,from sensing the communication voltage pulses generated by the sendingdevice.

In FIG. 10, the receiving device(s) of system 500 may identify theelapse time 1002 between communication voltage pulses 1010 a and 1010 b,and interpret that elapse time 1002 as, for example, a synchronizationsymbol. Likewise, the receiving device(s) of system 500 may identify theelapse time 1004 between communication voltage pulses 1010 b and 1010 c,and interpret that elapse time 1004 as, for example, a “1” symbol.Moreover, the receiving device(s) of system 500 may identify the elapsetime 1006 between communication voltage pulses 1010 c and 1010 d, andinterpret that elapse time 1006 as, for example, a “0” symbol. In somecases, the receiving device(s) of system 500 may detect thatcommunication voltage pulse 1010 d is not followed by anothercommunication voltage pulses within the threshold amount of time 1008,and may interpret that as an End of Frame (EOF) symbol. This particularexample is only illustrative, and it is contemplated that differentsymbols, different time delays and different sequences may be used,depending on the application.

In some examples, the devices of system 500 are continuously (althoughpossibly punctuated by blanking periods) listening for conductedcommunication signals. That is, the devices of system 500 may not sendout wake-up signals or establish specific communication connectionsbefore sending out conducted communication signals to other devices.Instead, the devices of system 500 may rely on synchronization pulses asa signal to the other devices of system 500 that the sending device issending a message. In some cases, an EOF symbol may be a signal that thesending device has communicated the entire message.

In the example of FIG. 10, time periods 1002, 1004, 1006, and 1008 aredepicted as being measured from a leading edge of each communicationvoltage pulse 1010. However, in other examples, time periods 1002, 1004,1006, and 1008 may be measured off of other features of communicationvoltage pulses 1010. For instance, the devices of system 500 may measuretime periods 1002, 1004, 1006, and 1008 from the trailing edge ofcommunication voltage pulses 1010. In still other examples, the devicesof system 500 may measure time periods 1002, 1004, 1006, and 1008 fromthe inflection point of communication voltage pulses 1010. Additionally,the devices of system 500 may not begin measuring a time period from afeature of communication voltage pulses 1010, for example the leadingedge, until the amplitude of the communication voltage pulse reaches athreshold level. In some cases, the devices of system 500 may measuretime periods 1002, 1004, 1006, and 1008 from a zero-crossing point ofcommunication voltage pulses 1010. These are just some examples.

FIG. 11 illustrates an example message 1100 that the devices of system500 may use to communicate data, commands and/or other information. Theillustrative message 1100 may be a command message that includes acommand for causing another device to take an action. Message 1100 mayinclude a synchronization field 1102, an address field 1104, a commandfield 1106, a payload field 1108, an error check field 1110, and an EOFfield 1112. Synchronization field 1102 of message 1100 may include oneor more synchronization symbols. As discussed previously, asynchronization symbol may indicate to receiving devices that a messageis being initiated.

Address field 1104 may include symbols that represent a relative deviceaddress (RDA). Each device of the system 500 may have an RDA thatuniquely identifies the device in the system 500. In some examples, theRDA may include three bits, allowing for eight devices with unique RDAs.In other examples, however, the address field 1104 may have greater orfewer RDA bits, as desired.

Address field 1104 may identify the devices to which the message isdirected. As described previously, in some examples, the devices ofsystem 500 may be constantly listening for conducted communicationsignals. Accordingly, each communication voltage pulse sent by a sendingdevice may be received by all devices of the system 500. However, once adevice has received a synchronization symbol and an RDA, the device mayattempt to match its own RDA (stored in local memory) with the receivedRDA. If the device determines that its RDA does not match the receivedRDA, the device may ignore the rest of the message. In some examples,this may simply mean that the device may not take action based on thecommand field 1106 in the message. In other examples, the device maybegin a blanking period or otherwise disable its circuitry that sensesfor conducted communication signals. If a device determines that thereceived RDA matches its own RDA, the device may continue to process themessage, e.g. take action according to a received command. In thismanner, the devices of system 500 may direct messages to specificdevices in the system. As used herein, the term ‘receiving device’ mayindicate any device that senses the conducted communication signals,e.g. all devices of the system within range of the conductedcommunication signals. This disclosure uses the term ‘intended device’to indicate the device to which a sending device directs a message.

In some examples, the devices of the system 500 may have multipleassociated RDAs. It may be desirable in some examples for the devices ofsystem 500 to direct messages to multiple devices. In examples where adevice has only a single unique RDA, the sending device would sendmultiple separate messages each with a different RDA. However, inexamples where devices have more than one associated RDA, at least oneof the associated RDAs may not be unique. As one example, two separatedevices may have their own associated unique RDAs and a second,non-unique RDA which is the same for both devices. Accordingly, todirect a message to both of the devices, a sending device would needonly to send a single message with the second RDA, as the second RDA isassociated with both the devices. In this manner, a device may generallyhave one unique RDA and any suitable number of non-unique RDAs which arealso associated with one or more other devices to facilitatecommunication from one device to a number of devices. In at least someexamples, each device may have an RDA that is the same across alldevices in the system.

Accordingly, when a device sends a message with such an RDA, the messageis directed to all of the devices in the system 500. Although thedevices of system 500 have been described as having RDAs that are threebits in length, other example systems may have RDAs that have more orless bits. The specific length of an RDA may be chosen according to thenumber of unique devices in a system and the desired combinations ofdevices for the purpose of directing messages.

In one example message, command field 1106 may include a three bitcommand. However, in other examples, the command field may be any numberof bits. The command field may represent an instruction by the sendingdevice for the receiving device or devices to perform one of a number ofpredefined commands.

Payload field 1108 may include one or more bits of data that the sendingdevice includes in the message. For some commands, the receiving devicemay need the data, address and/or other information included in payloadfield 1108 to take the desired action based on the command received incommand field 1106. In some examples, payload field 1108 has a range ofpossible sizes, such as zero bits to twenty-four bits. However, in otherexamples, payload field 1108 may be any other suitable size.Alternatively, the payload field may have a fixed length, which in somecases may depend on the command specified in command field 1106. Forinstance, for a “Read Byte” command, payload field 1108 may be a ninebit address. However, for a “Write Byte” command, payload field 1108 mayinclude a nine bit address and an eight bit data field for a total ofseventeen bits.

Error check field 1110 may include an error checking code, which thereceiving device may use to determine if the received message wascorrupted during transmission. For example, the contents of error field1110 may include bits that are used by the receiving device in a paritycheck scheme, a checksum scheme, a cyclic redundancy check scheme,and/or some other type of error checking scheme. Error check field 1110may also include an error correction scheme. For example, error checkfield 1110 may include hamming, Reed-Solomon or other correction codes.

In some examples, if the receiving device determines that the messagewas corrupted, the receiving device may send a command to the sendingdevice to re-send the message. However, in some example systems, theremay not be a command to request that the sending device resend themessage (as is missing in Table 1). In such examples, if the corruptedmessage was a command message, the receiving device may take no actionand send no response message (described below with respect to FIG. 12).After not receiving a response message within a predetermined period oftime, the sending device may resend the command message. If thecorrupted message was a response message, the device that sent thecommand message may simply send the command message again to triggeranother response message.

EOF field 1112 may simply be an EOF symbol that the sending deviceincludes to indicate the end of the message. As described above, in someexamples, the receiving device may identify an EOF symbol based on alack of a communication voltage pulse for a threshold period of time(rather than a specific time period between two communication voltagepulses). In such examples, EOF field 1112 may simply represent a lack ofa generated communication voltage pulse for a threshold length of timeby the sending device, as opposed to sending any affirmative signals orbits.

Table 1 below lists some example commands that a device of system 500may perform, along with the three bits that identify the command(expressed in hexadecimal in Table 1):

TABLE 1 Command CMD CMD Type # RDA Payload Response Description Reset0x0 RDA None ACK Cause a System Reset. ID 0x3 new 24-bit Unique ACK ifAssign the RDA in the (pairing) RDA device ID serial # message to thedevice if the [23:0] = serial # matches; 24-bit payload matches the elsenone serial number. Read Byte 0x4 RDA 9-bit Byte if ok, Read a singledevice byte [8:0] = address else none from the address specified. Write0x6 RDA 17-bit Ack if ok, Write a single device byte to Byte [16:8] =address else none the address specified. [7:0] = data Read 0x7 RDA17-bit Multiple Read up to 8 consecutive Multiple [16:8] = address Bytesif bytes from the device. The [7:0] = count ok, else first byte in thepayload will none be the value from the address. The second byte will befrom the next consecutive and so on. ACK 0x1 RDA 0-bit Acknowledge anID, PING, or WRITE_BYTE command. PING 0x1 RDA 0-bit ACK Used for fasterpolling of devices. DEBUG 0x5 Global 9-bit Byte if ok, This specialcommand may [8:0] = address else none be used for identifying twodevices with a global RDA. The turn-around time for this command will bebased on the last 6-bits of the device's serial number. The turn- aroundtime will be equal to 20 ms × serial_number[5:0]. This allows twodevices to respond w/o corrupting each other. OPEN 0x2 RDA n/a n/a Openslot for a future command.The “command type” column lists the names of the various commands that adevice may include in the illustrative message 1100 of FIG. 11. The “CMD#” column references a specific three bit code used to uniquely identifyeach command. In Table 1, the three bit code is expressed in ahexadecimal format. Accordingly, 0x0 may be expressed in binary as 000,0x1 may be expressed as 001, 0x2 may be expressed as 010, and so on.When an intended receiving device receives the three bit command, thatdevice may match up the received three bits to the commands expressed inTable 1, and may take the requested action based on the identifiedcommand. In some systems, more commands may be defined and each commandmay be identified by a greater number of bits. The “RDA” columnidentifies the type of RDA that the sending device needs to include inthe messages for each command. The “CMD Payload” column identifies thespecific data that the sending device needs to include in the messagefor each command. The “Response” column describes the type of responsethat the intended receiving device (or devices) will return for eachcommand. Finally, the “Description” column gives a general descriptionof the function of each command. Descriptions of each of the commandslisted in Table 1 are described below:

Reset Command

After a device receives a “Reset” command (and has an RDA that matchesthe RDA specified in the RDA field of the Reset command), the receivingdevice performs a reset. In one example, the receiving device maytemporarily cut power to its processing module and/or memory circuit.This power cycle may cause the memory circuit to lose one or more storedparameters, for example if the memory circuit includes at least onevolatile memory portion. In some cases, memory circuit may include atleast one non-volatile memory portion. In such examples, the device mayretain one or more parameters that are stored in the non-volatile memoryportion. While power cycling is one way to perform a reset, it iscontemplated that any suitable method may be used to reset the receivingdevice.

ID (Pairing) Command

After receiving the “ID (pairing)” command, a receiving device mayassociate itself with a specific RDA. In one example, each receivingdevice may have a unique identifier stored in a non-volatile memory. Asone example used herein, the unique identifier may be a serial numberassociated with a device, such as at the time of manufacture orthereafter. Before associating itself with any RDA, a device may receiveand process all messages as if the device were the intended receivingdevice. If a message including an ID (pairing) command is received, thereceiving device may determine if the serial number specified in thepayload field matches its own serial number. If the serial numbersmatch, the receiving device may associate the RDA specified in theaddress field of the message with itself, and store the RDA in its localmemory (non-volatile or volatile memory). In some examples, this pairingmay only be done once for each device for the life of the device, whilein other cases, this pairing may be done at any suitable time. In someexamples, a device that is not to be a part of a medical device systemfor delivering electrical stimulation therapy to a patient may issue oneor more ID (pairing) commands to the medical devices that are to be partof a medical device system for delivering electrical stimulation therapyto a patient. For example, a programmer device may issue ID (pairing)commands to each medical device of a medical device system to assignRDAs to each of the medical devices. The programmer device may only beused once before or at the time of implantation of the medical devicesinto a patient or may only be used at limited times, such as in amedical office setting, for retrieving information from the medicaldevices of the system or changing settings of the medical devices.Accordingly, it may not be the case in some examples that the devicesending ID (pairing) commands is also a device that communicates withdevices of the medical device system in order to deliver electricalstimulation therapy to the patient.

Read Byte Command

If a receiving device receives a “Read Byte” command (and has an RDAthat matches the RDA that is specified in the RDA field of the Read Bytecommand), the receiving device reads the data byte stored at the addressincluded in the payload field of the message, and sends the requesteddata byte to the sending device.

Write Byte Command

If a device receives a “Write Byte” command (and has an RDA that matchesthe RDA that is specified in the RDA field of the Write Byte command),the receiving device may write the data byte specified in the payloadfield of the message to the address specified in the payload field ofthe message. In the example shown, nine bits of the payload field mayspecify a memory address, and eight bits may specify the data to bewritten. In some examples, the payload field of the message may bestructured differently.

Read Multiple Command

If a device receives a “Read Multiple” command (and has an RDA thatmatches the RDA that is specified in the RDA field of the Read Multiplecommand), the receiving device may read multiple data bytes from itsmemory, and send the multiple data bytes to the sending device. In oneexample, the payload field of the message may specify a starting memoryaddress as well as number of bytes. The receiving device may read databytes beginning at the specified starting memory address and continuingreading subsequent memory addresses until the specified number of byteshave been read, and then send the requested data bytes to the sendingdevice. In some examples, the receiving device may read from consecutiveaddresses that increase relative to the specified starting address. Inother examples, the receiving device may read from consecutive addressesthat decrease relative to the specified starting memory address. Instill other examples, the sending device may specify whether thereceiving device should read and send data from memory addresses thatincrease or decrease relative to the specified starting memory address.

ACK and Ping Commands

If a device receives an “ACK” command (and has an RDA that matches theRDA that is specified in the RDA field of the ACK command), the messagedoes not have a payload field, and the receiving device may take noaction based on the command. If a receiving device receives a “PING”command (and has an RDA that matches the RDA that is specified in theRDA field of the PING command), the receiving device may simply respondwith a message having an “ACK” command. As with the “ACK” command, amessage that includes a “PING” command may not have any payload field.

Debug Command

The “DEBUG” command may be sent in a message with an RDA that is sharedby at least two devices (e.g. a global RDA such as 111). The DEBUGcommand may include a payload field that specifies a memory address.Each intended receiving device (based on RDA) may read data stored atthe specified memory address and send the data back to the sendingdevice. Each intended receiving device may send the data at differenttimes. In one example, each intended receiving device may wait adifferent number of milliseconds before sending the data so the intendeddevices do not send the read data at the same time. In some examples,each device may determine to wait time based on an equation with thelast six numbers of the device's serial number as one variable. Forexample, each device may wait for twenty microseconds multiplied by thelast six bits of the device serial number.

Open Command

Finally, there may be an undefined command associated with a uniquethree bit identifier (e.g. 0x2). In Table 1, the command is labeled“OPEN.” Devices may later be programmed such that the OPEN commandcauses the intended device to take some action. For instance, the OPENcommand may be used to assign and/or un-assign non-unique RDAs to asingle device to allow for more complex multi-device messaging withoutaffecting associated unique RDAs. This is just one example.

Turning now to FIG. 12, FIG. 12 illustrates an example message 1200 thatthe devices of system 500 may use to communicate data and otherinformation. Message 1200 may be, for example, a response message sentby a receiving device in response to receiving a command message. In theexample shown, message 1200 may include a synchronization field 1202, anaddress field 1204, a response field 1206, a payload field 1208, anerror check field 1210, and/or an EOF field 1212. Synchronization field1202, address field 1204, payload field 1208, error check field 1210,and EOF field 1212 may be similar to synchronization field 1102, addressfield 1104, payload field 1108, error check field 1110, and EOF field1112 as described with respect to FIG. 11.

One difference between illustrative message 1200 and illustrativemessage 1100 is that message 1200 has response field 1206 instead of acommand field such as command field 1106. As discussed above, a messagewith a command field may include a command for an intended receivingdevice to take some action. A receiving device may send a responsemessage with a response field in response to a command message. Responsefield 1206 (and sometimes the payload field) may include some sort ofexplicit response to the received command message. For example, afterreceiving a message with the ID (pairing) command, and after theintended receiving device matches its serial number to the serial numberin the payload field of the received ID (pairing) command message, theintended receiving device may send a response message back to thesending device. In some instances, the response message may include areference to the ACK command in response field 1206. In some cases, theresponse message may not include anything in the payload field 1208.However, if the receiving device does not match the received serialnumber to its own serial number, the receiving device may take no actionand send no response message back to the sending device.

If an intended receiving device receives a “Read Byte”, “Read Multiple”,or “DEBUG” command, the receiving device may read the requested datafrom one or more memory address. In response to receiving any of thecommands, the intended receiving device may send a response message thatdiffers from response message 1200. For example, the response messagemay include a synchronization field 1202, an address field 1204, apayload field 1208, an error checking field 1210 and/or an EOF field1212. This response message may lack response field 1206. Payload field1208 of such response messages may contain the requested data read fromthe one or more memory addresses. In other examples, the responsemessage may also include response field 1204.

If an intended receiving device receives a “Write Byte”, “Reset”, or“Ping” command in a command message, the intended receiving device maysend a response message back to the sending device with an ACK commandin response field 1206. This response message may have no payload field1208.

Although in the above description, in some examples, the commandmessages and response messages may omit one or more fields of a message,this may not be true in all cases. For instance, as described above, ina command message having a “PING” command, the command message may notinclude payload field 1108. However, in other examples, the commandmessage may include a blank payload field. For example, the payloadfield may include all zeroes. In such messages, the message may belonger. However, each message may be of a constant size (e.g. samenumber of bits), which may allow for a less complex implementation forprocessing messages. It additionally may not be the case that payloadfields 1108 and 1208 differ based on command types. For instance,payload fields 1108 and 1208 may have a fixed size. The fixed size maybe based on a maximum amount of data to be transferred in a singlemessage. In situations where a message does not need the entirety of apayload field for the data to be transmitted, the remainder of thepayload field may be blank, for example filled in with zeroes. Again,this may result in messages with constant lengths.

In the above described manner, each field described in FIGS. 11 and 12may be comprised of a set of communication pulses. For instance, inorder to convey information each field may include a plurality ofcommunication pulses in order to convey multiple bits to comprise theconveyed information. However, in some examples, a single communicationpulse may be enough to convey information to a receiving device.Accordingly, in some instances, a “set” of communication pulses mayinclude just a single communication pulse, while in other instances a“set” of communication pulses may include multiple communication pulses.

FIG. 13 is a flow diagram of an illustrative method 1300 that may beperformed by a receiving device. A receiving device may receive acommand message, as shown at 1302. The device may determine if the RDAcontained in the address field 1104 of the command message matches theunique RDA of the receiving device, as shown at 1304. If the RDA in thecommand message does match the unique RDA of the receiving device, thereceiving device may execute the command reference in the command field1106 of the command message, as shown at 1306, and then exit as shown at1350. If the RDA in the command message does not match the unique RDA ofthe receiving device, for example because the RDA in the command messageis different from the unique RDA of the receiving device or thereceiving device does not yet have an associated unique RDA, thereceiving device may determine if the command in the command field isthe ID (pairing) command, as shown at 1308. If the command is the ID(pairing) command, the receiving device may determine if the payloadfield of the message matches the unique serial number of the receivingdevice, as shown at 1310. If the payload field does match the uniqueserial number of the receiving device, the receiving device may set itsRDA equal to the RDA in the address field of the message, as shown at1312. If the payload field does not match the unique serial number ofthe receiving device, the receiving device may ignore the command, asshown at 1314, and exit as shown at 1350.

If the command is not the ID (pairing) command, the receiving device maydetermine if the RDA of the command message is one of the receivingdevice's non-unique RDAs, as shown at 1316. For example, as describedpreviously, each receiving device may have a number of associatednon-unique RDAs in addition to each receiving device's unique RDA. Ifthe RDA of the command message is not one of the receiving device'snon-unique RDAs, the receiving device may ignore the command, as shownat 1320 and exit as shown at 1350. However, if the RDA of the commandmessage is one of the receiving device's non-unique RDAs, the receivingdevice may execute the command, as shown at 1318, and exit as shown at1350.

In some examples, a receiving device may respond differently to commandmessages based on whether the RDA of the command message is one of thereceiving device's non-unique RDAs or the receiving device's unique RDA.For instance, in some instances, a receiving device may perform thefunctions described above with respect to Table 1 for a given commandmessage if the command message included the receiving device's uniqueRDA. However, the receiving device may behave differently to one or moreof the commands if the RDA of the command message is one of thereceiving device's non-unique RDAs. In one example, one of thenon-unique RDAs of a receiving device may be a global RDA, e.g. an RDAthat is shared by all of the devices of a system. If the receivingdevice receives a command message with the global RDA, and the commandis a Write Byte command or a Reset command, the receiving device mayperform those functions but may not send a response message with an ACKresponse. Additionally, if the command is a Ping, Read Byte, or ReadMultiple command, the receiving device may ignore these commands. Inother systems, a receiving device may execute the commands in thesedifferent ways only if the command message includes one of the receivingdevice's non-unique RDAs and the receiving devices has not yet set itsunique RDA. These are just examples.

In some examples, a device may be preprogrammed with an RDA. Forinstance, a processing module or memory module may be preprogrammed witha specific RDA so that when the processing module or memory module isincorporated into a device, the device then has an RDA. In otherexamples, a device may be connected directly to a programming device,and the programming device may set the RDA of the device. In suchexamples, a device may not include a specific ID command. For example,the device may not recognize and ID command and may not change or set anRDA after receiving an ID command. In such examples, instead ofperforming a method such as described with respect to FIG. 13, includingdetermining whether a received command is an ID command, the device maysimply ignore any message that does not include an RDA that is equal toits own RDA.

FIGS. 14 and 15 illustrate various timing schemes of sending commandmessages and response messages. FIG. 14 displays a command message 1402and a response message 1404 on a time line 1406. Command message 1402and response message 1404 are separated by turn-around time 1408. Insome examples turn-around time 1408 may be one half of a millisecond.However, in other examples, turn-around time 1408 may be one-quarter,three-quarters, one, or two milliseconds, or any other suitable lengthof time. In some cases, the turn-around time 1408 may be a fixed or avariable value, which may depend on factors such as, for example, systemnoise, signal-to-noise ratio, signal strength, processing power of thereceiving device, battery level of the receiving device, number ofreceiving devices in the overall system, etc.

As detailed above with respect to FIGS. 6A-6D, the command message andthe response message may each be communication using a plurality ofspaced communication pulses each having an amplitude and a pulse width.It is contemplated that the amplitude and/or pulse width of thecommunication voltage pulses used to send the command message 1402 maybe different from the amplitude and/or pulse width of the communicationvoltage pulses used to send the response message 1404.

More generally, when a first implantable medical device sends a firstmessage (e.g. command message 1402 or response message 1404) from thefirst implantable medical device to a second implantable medical device,the plurality of spaced communication pulses may have a first amplitudeand a first pulse width. When the second implantable medical devicesends a second message (e.g. response message 1404 or command message1402) from the second implantable medical device to the firstimplantable medical device, the plurality of spaced communication pulsesmay have a second amplitude and a second pulse width. In some cases, thefirst amplitude and the second amplitude may be substantially the same(e.g. +/−10%), but the first pulse width and the second pulse width maybe substantially different. In some cases, the second pulse width may be2, 3, 4, 5 or more times the first pulse width. In some cases, the firstamplitude and the second amplitude may be substantially different, andthe first pulse width and the second pulse width may be substantiallythe same (e.g. +/−10%). In some cases, the second amplitude may be 2, 3,4, 5 or more times the first amplitude. In some cases, the firstamplitude and the second amplitude may be substantially different, andthe first pulse width and the second pulse width may be substantiallydifferent.

In some instances, the first implantable medical device may be animplantable subcutaneous cardioverter, and the second implantablemedical device may be an implantable leadless cardiac pacemaker. This isjust one example. However, because of the different locations of each ofthese devices in the body, as well as other factors such as batterycapacity, the amount of energy that can be provided in the communicationpulses without causing capture and/or without causing excessive batterydrain, may be substantially different. For these and other reasons, theamplitude and/or pulse width of the communication pulses emitted by eachof the devices may be different.

FIG. 15 illustrate various timing schemes for implementing commandmessages and response messages. An example electrocardiogram 1500 isshown that includes a number of cardiac cycles, shown by QRS waves 1502a-1502 c, and command and response message pairs 1504 and 1515. Thedevices of system 500, in addition to sensing for conductedcommunication signals as described previously, may also sense forcardiac electrical activity such as intrinsic and/or paced heartbeats.In some examples, intrinsic and/or paced heartbeats may be detected byidentifying QRS waves 1502 a-1502 c of the electrocardiogram 1500. Inanother example, intrinsic and/or paced heartbeats may be detected byidentifying the R wave of the QRS waves 1502 a-1502 c of theelectrocardiogram 1500. Regardless of how the intrinsic and/or pacedheartbeats are detected, the devices of system 500 may be configured tobegin a message blanking period around detected QRS waves 1502 a-1502 c,for example message blanking periods 1510 a-1510 c. The devices ofsystem 500 may be configured to not send any command or responsemessages during such message blanking periods 1510 a-1510 c. Saidanother way, the devices of system 500 may be configured to allowcommunication between the devices of system 500 except during theblanking periods.

In some cases, the blanking periods 1510 a-1510 c are initiated after anintrinsic heartbeat is detected, and may extend for a period of timethereafter. For example, blanking periods 1510 a-1510 c may be initiatedafter detecting a P wave of a heartbeat signal. In other examples, theblanking periods 1510 a-1510 c may not begin until after the S wave ofthe QRS waves 1502 a-1502 c are detected. In still other examples, theblanking periods 1510 a-1510 c may begin when the corresponding R waveof the QRS waves 1502 a-1502 c is detected.

In some examples, a device may detect heartbeat (e.g. a QRS wave) whilein the process of sending a message, as illustrated with QRS wave 1502 coverlapping command message 1505 of a command and response message pair.In such examples, the sending device may cease sending the message upondetection of QRS wave 1502 c and initiation of blanking period 1510 c,as indicated in FIG. 15. Although in other examples, the sending devicemay continue sending the message. In any of these examples, it ispossible that the message may not be properly received, either becausethe message was cut short or because the signal-to-noise ratio of thetransmission may be low due to the “noise” caused by the QRS wave 1502c. Once blanking period 1510 c has passed, the device may send thecommand message a second time, as indicated by command and responsemessage pair 1515. In a similar manner, if a device sending a responsemessage detects a heartbeat during the communication of the responsemessage, the device may resend the response message a second time afterthen end of a blanking period. Although in other examples, the sendingdevice may instead cease sending the response message and not resend theresponse message after the end of a blanking period. Accordingly, thedevice that sent the command message may not receive within apredetermined amount of time. In such examples, the device that sent thecommand message may resend the command message a second time which wouldprompt a another response message from the receiving device, asdescribed previously with respect to FIGS. 11 and 12 and table 1.

Devices of system 500 may additionally detect stimulation pulses,represented illustratively by stimulation pulse 1506. In such examples,the devices may be configured to implement a blanking period afterdetecting a stimulation pulse 1506, such as blanking period 1512. In atleast some examples, blanking period 1512 may be longer than any ofblanking periods 1510 a-c, as shown in FIG. 15. However, in otherexamples, even if blanking period 1512 is longer than any of blankingperiods 1510 a-c, blanking period 1512 may end at a similar time afterpaced beat 1502 b as blanking periods 1510 a and 1510 b end afterintrinsic beats 1502 a and 1502 c. The devices of system 500 may dealwith detecting stimulation pulse 1506 during transmission of a messagein a similar manner to detecting QRS waves 1502 a-15102 c during messagetransmission. For example, the devices may be configured to re-send anymessage or portion of a message pair after the end of blanking period1512 that overlaps in time with a stimulation pulse 1506. Again, in someexamples, the devices may continue sending the message even afterdetection of a stimulation pulse 1506, but in other examples the devicesmay cease sending the message upon detection of a stimulation pulse1506.

In examples where a device begins blanking period 1512, the device maynot also institute a blanking period in response to detecting a QRSwave, such as blanking period 1510 b. However, in other examples, thedevice may also institute blanking period 1510 b in addition to blankingperiod 1512. In such examples, the device may not send or re-send anymessages until both blanking periods 1512 and 1510 b have ended.

FIG. 16 is a flow diagram of an illustrative method that may beimplemented by an implantable medical device, such as shown in FIGS.1-4, or a medical device system such as shown in FIG. 5. Although themethod of FIG. 16 will be described with respect to LCP 100 and MD 300,the illustrative method of FIG. 16 may be performed using any suitablemedical device or medical device system.

According to the method depicted in FIG. 16, a first medical device,such as MD 300, may be implanted within a patient, such as if MD 300 isan ICP, an ICD, an S-ICD, or may be disposed in proximity to thepatient, such as if MD 300 is an external medical device. MD 300 may bepart of a medical device system along with a second medical device, suchas LCP 100. In such a medical device system, one or more medicaldevices, such as MD 300 and/or LCP 100, may be configured to sensecardiac electrical signals, as shown at 1602. The one or more medicaldevices may be further configured to determine occurrences of intrinsicheartbeats, as shown at 1604. The one or more medical devices may befurther configured to provide a blanking period after each occurrence ofan intrinsic heartbeat, as shown at 1606. The one or more medicaldevices may also be configured to allow communication between the one ormore medical devices and one or more other medical devices except duringthe blanking periods, as shown at 1608.

Those skilled in the art will recognize that the present disclosure maybe manifested in a variety of forms other than the specific examplesdescribed and contemplated herein. For instance, as described herein,various examples include one or more modules described as performingvarious functions. However, other examples may include additionalmodules that split the described functions up over more modules thanthat described herein. Additionally, other examples may consolidate thedescribed functions into fewer modules. Accordingly, departure in formand detail may be made without departing from the scope and spirit ofthe present disclosure as described in the appended claims.

What is claimed:
 1. A method for communicating between a plurality ofmedical devices, wherein at least one of the plurality of medicaldevices is an implantable medical device, the method comprising: sensingcardiac electrical signals; determining occurrences of intrinsicheartbeats; providing a blanking period after each occurrence of anintrinsic heartbeat; and allowing communication between a first medicaldevice and a second medical device except during the blanking periods.2. The method of claim 1, further comprising determining occurrences ofpacing pulses, and providing a blanking period after each occurrence ofa pacing pulse.
 3. The method of claim 1, wherein the first medicaldevice and the second medical device are both implantable medicaldevices.
 4. The method of claim 2, wherein providing a blanking periodafter each occurrence of an intrinsic heartbeat and each occurrence of apacing pulse comprises providing a first blanking period after eachoccurrence of an intrinsic heartbeat and providing a second blankingperiod after each occurrence of a pacing pulse, wherein the secondblanking period is longer than the first blanking period.
 5. The methodof claim 1, wherein communication between the first medical device andthe second medical device comprises communicating one or more messagesof predefined lengths.
 6. The method of claim 5, wherein communicatingbetween the first medical device and the second medical device comprisescommunicating a first message from the first medical device to thesecond medical device via conducted communication.
 7. The method ofclaim 6, wherein if an intrinsic heartbeat occurs while the firstmedical device is sending the first message from the first medicaldevice to the second medical device via conducted communication,providing a first blanking period and resending the first message afterexpiration of the first blanking period.
 8. The method of claim 7,wherein if a pacing pulse occurs while the first medical device issending the first message from the first medical device to the secondmedical device via conducted communication, providing a second blankingperiod and resending the first message after expiration of thecorresponding second blanking period, wherein the second blanking periodis longer than the first blanking period.
 9. The method of claim 6,wherein communicating between the first medical device and the secondmedical device comprises sending a second message from the secondmedical device to the first medical device via conducted communicationin response to the first message.
 10. The method of claim 9, wherein ifthe first medical device fails to receive the second message within apredetermined period of time after sending the first message, resendingthe first message.
 11. A medical device, comprising: a communicationmodule for communicating with an implantable leadless cardiac pacemakerthrough body tissue; a controller operatively coupled to thecommunication module, wherein the controller is configured to: identifyintrinsic heartbeats; provide a blanking period after each occurrence ofan intrinsic heartbeat; and communicate with the implantable leadlesscardiac pacemaker via the communication module only during times betweenthe blanking periods.
 12. The medical device of claim 11, wherein thecontroller is further configured to: identify pacing pulses; and providea blanking period after each occurrence of a pacing pulse.
 13. Themedical device of claim 11, wherein the communication modulecommunicates with the implantable leadless cardiac pacemaker viaconducted communication.
 14. The medical device of claim 13, wherein thecommunication module communicates with the implantable leadless cardiacpacemaker by providing a plurality of communication pulses through bodytissue, wherein each of the communication pulses is below a capturethreshold of a heart.
 15. The medical device of claim 12, wherein thecontroller is further configured to: provide a first blanking periodafter each intrinsic heartbeat; and provide a second blanking periodafter each pacing pulse, wherein the second blanking period is longerthan the first blanking period.
 16. A medical device system configuredto deliver electrical stimulation therapy to a heart of a patient, thesystem comprising: an implantable leadless cardiac pacemaker configuredto deliver electrical stimulation therapy to the heart of the patient; amedical device communicatively coupled to the implantable leadlesscardiac pacemaker and configured to: identify intrinsic heartbeats; andinitiate communication messages between the medical device and theimplantable leadless cardiac pacemaker only at times other than duringthe identified intrinsic heartbeats.
 17. The medical device of claim 16,wherein the medical device is further configured to: identify pacingpulses; and initiate communication messages between the medical deviceand the implantable leadless cardiac pacemaker only at times other thanduring the identified intrinsic heartbeats and the identified pacingpulses.
 18. The medical device system of claim 17, wherein the medicaldevice is further configured to: provide a blanking period after eachidentified intrinsic heartbeat and each identified pacing pulse; andallowing communication messages to be initiated between the medicaldevice and the implantable leadless cardiac pacemaker except during theblanking periods.
 19. The medical device system of claim 17, wherein themedical device is further configured to: provide a first blanking periodafter each identified intrinsic heartbeat; and provide a second blankingperiod after each identified pacing pulse, wherein the second blankingperiod is longer than the first blanking period.
 20. The medical devicesystem of claim 16, wherein if the medical device identifies anintrinsic heartbeat while communicating a message between the medicaldevice and the implantable leadless cardiac pacemaker, the medicaldevice is further configured to provide a first blanking period andresending the message after expiration of the first blanking period.